In general, the exhaust gas of a diesel engine is known to contain particulate matters (hereinafter, referred to as PM (Particulate Matter)) containing a carbon as a main component, and to cause air pollution. For this reason, various kinds of devices and methods are proposed for trapping and removing these particulate matters from exhaust gas. For example, (i) a method in which PMs which have been trapped are oxidized and burned by increasing the temperature of a diesel particulate filter (hereinafter, referred to as “DPF”) by forcibly injection-supplying a fuel, (ii) a method in which NO2 is produced from NO in the exhaust gas, and the PMs are oxidized by the NO2 (for example, International Application Japanese-Phase Publication No. 2002-531762 (document 1)), and (iii) a method in which PMs are oxidized using the catalyzed DPF (for example, Japanese Unexamined Patent Application Publication No. Hei 6-272541 (Document 2), and Japanese Unexamined Patent Application Publication No. Hei 9-125931 (Document 3)) are proposed.
However, in the method (i), there were problems that reduction in fuel cost performance was caused because of the forcible injection-supply of the fuel, and also that the PMs rapidly burned, resulting in the breakage of the DPF due to a rapid change in temperature derived from the reaction heat generated during the rapid burning of the PMs. Moreover, in the method (ii), there was a problem that the oxidation rate of the PMs by NO2 was insufficient, and thereby it was difficult to completely oxidize and remove the PMs emitted from the engine. Furthermore, in the method (iii), there was a problem that, both the catalyst and the PMs were solid, so that the catalyst and the PMs were not sufficiently in contact with each other, resulting in insufficient oxidation reaction of the PMs.
Recently, a technology has been disclosed (for example, International Application Japanese-Phase Publication No. 2005-502823 (Document 4)). The technology is to oxide and treat PMs using ozone (O3) having a strong oxidizing power as compared to NO2. In the method described in the Document 4 by which the exhaust gas of a diesel engine is after-treated, the PMs trapped in the particulate filter are oxidized and removed by mounting, on the upstream of the particulate filter, a device for producing ozone or NO2, which serve as an oxidant, from the exhaust gas using a plasma in order to selectively use ozone and NO2 at low temperatures, and NO2 at high temperatures corresponding to the temperature of the exhaust gas.
However, in the method described in Document 4, ozone is formed from oxygen, which is a component of the exhaust gas, using the plasma, and the exhaust gas containing NOX and the like is introduced into the particulate filter together with the formed ozone. Thereby, the ozone having a strong oxidizing power is reacted with NOX and the like in the exhaust gas, and is consumed before entering the particulate filter. As a result, the amount of the ozone to be used for the oxidation and removal of the PMs is reduced, and thereby a sufficient efficiency of purification is not obtained. Accordingly, there was a problem that the oxidation rate of the PMs was reduced.